OP SOUTHERN INDIA. 411 



the number of recent species at somewhat near 500, that of the fossil is certainly 

 four times as large. However, the different families are not equally represented in 

 the different formations. The Babulidje, Anomiid^, and Pectinid^. already occur 

 in palaeozoic strata ; but only the species of the last are numerous, represented by 

 several peculiar types which again soon disappear. The Spondylidje and Ostreid/e 

 are only known with sufficient certainty since the beginning of the mesozoic epoch. 

 It is not improbable that most of the families have the maximum of theii- deve- 

 lopment during the cretaceous period : with the Txabvlidm and Ostueibje it is 

 certain, with the Pectinid^ and Spondylid^ probable, wliile the species of fossil 

 AxoMiiD/E are as yet few, and still fewer are the Placunid-e. 



XLI . Family,— E A D VLIBJE. 



The mantle margins of the animal of Badula are entirely disunited, thickened, 

 provided with several irregular rows of elongated filaments of various sizes; the 

 inner edge of the mantle is thin, produced all round, closed for some distance over 

 the oral and less so over the anal region ; on the internal side of each mantle leaf it 

 forms an open loose bag in which the gills are placed. These are narrowly attached 

 at the antero-inferior margin of the adductor, are of an elongately oval shape and 

 loosely suspended in the bag ; they consist of very fine, rather distant fillets. TJie 

 palps are transversally elongated, short, internally striated ; the two leaves of each 

 pair are anteriorly united for a great part, and the pairs themselves are again united 

 at the base across the front. The body is usually rather large, ovately elongated ; 

 the foot anterior, placed some distance below the palps, elongated, sub-cylindrical, 

 slightly dilated at the end, grooved above, and with a byssal gland at the base ; the 

 pedal muscles are not developed; the foot only appears as a narrowly produced 

 thickening of the skin of the body. The adductor is placed rather posteriorly, not 

 far from the hinge ; it is of an oval shape ; the rectum is grown to the posterior 

 side of the adductor and has a short, free, tubular termination. 



I have examined the animals of two species, one of Padula, as restricted, and 

 the other of the sub-genus llantellum : they are quite similar. The large deve- 

 lopment of the internal thin layer of the mantle, forming an open bag, appears to 

 be destined for hatching the eggs, and is no doubt also an important organ for 

 retaining w^ater while swimming. There is a distinct byssal gland present, being 

 able to spin a byssus, if required, but its secretion does not appear to be permanent. 



The shells are obliquely oval, inflated or compressed, gaping or closed, always 

 equivalve and free ; hinge margin on each side produced into short, unequal ears ; 

 hinge area more or less developed, with a median triangular pit for the attachment 

 of a partially internal ligament, sometimes with a byssal and pedal gape below the 

 anterior ears. 



By far the greatest number of the species of BadulidjE is extinct. Those 

 belonging to Liniea first occur in the Trias and continue up to the present time, 

 but in very small numbers ; the species of Raclula are, on the contrary, already 



